1. Field of the Invention
This invention relates to a device and method for bypassing a telephone system such as an ACD (automatic call distributor) or a PBX (private branch exchange) control unit in the event of a system failure or a power failure. The invention more specifically allows telephones connected to the ACD or PBX to be used for incoming and outgoing calls during a control unit or power failure by connecting the telephones to a T1 span trunk (or other digital telephone line) to the central office.
2. Description of the Prior Art
The T1 digital transmission system, carrying DS-1 signals, is the primary telephone digital communication system in North America. Providing full-duplex transmission at 1.544 megabits per second (Mbps), a single T1 facility (also called a T1 span) normally handles 24 multiplexed 64 Kilobytes per second (KBps) channels for the transmission of digitized voice, or as many as several hundred lower-speed data signals on an "unchannelized" basis by means of a T1 multiplexer.
The T1 signal is a bipolar pulse train with a 50% duty cycle. Each time slot may contain a single pulse of one-half the time slot duration. If a pulse is present, the time slot represents a 1 bit (binary digit); if no pulse is present, the time slot represents a 0 bit. Consecutive pulses are expected to be of opposite polarity; thus, consecutive pulses of the same polarity (called a "bipolar violation" or BPV) indicate a transmission error.
Common carriers such as AT&T and the regional Bell Operating Companies lease various T1 services, both terrestrial and satellite. Within a local telephone company, T1 signals are frequently transmitted over standard twisted-pair copper cable circuits. Repeaters regenerate the signal every mile or so, ensuring reliable transmission over long distances even if the transmission facilities themselves are poor. In addition, T1 signals may be transmitted using digital microwave radios, fiber optic systems, and coaxial cable modems.
PBX's and ACD's (see FIG. 1 depicting a conventional analog bypass unit) are well known in the art. A typical ACD 10 includes a system controller 12 (which may be for instance a multiprocessor computer) for performing call processing and administrative functions.
The system controller controls a switching subsystem 14 which is a digital switching matrix. The switching matrix supports analog trunks 26, T-1 digital trunks (not shown), and telephone sets 20a, 20b, 20c, and 20d through a telephone set interface card 16 (see solid line denoting normal ACD operation). The telephone sets 20a, 20b, 20c, and 20d connected to a typical ACD 10 may be called agent telephone sets, since the telephone users are often sales or reservation agents.
Given the critical nature of their operation, in the event of a failure, most high-volume call centers want to bypass the ACD 10 for as many trunks 26 (i.e. lines) as there are agents available to take the calls. Critical component redundancy and universal power supplies are too costly for many call-center installations. Therefore, analog trunk bypass mode (dotted lines) by a trunk bypass unit 28 in the main distribution frame 24 has been the usual prior art method for failure backup. Analog trunk bypass connects analog telephone access trunk lines 26 directly to analog telephones 30a, 30b in the case of a system or power failure.
An alternative conventional bypass system (not shown) connects ground/loop start trunk lines at a ground/loop start card 31 to ordinary telephones via a manual or remotely activated relay transfer (not shown). There is a one-to-one association between incoming trunk lines and telephone sets, and this association is fixed.
Most ACD's and PBX's feature digital switch designs that offer direct trunk connection to digital T1 services. For most high-volume call centers, the cost of digital T1 trunks is significantly less than the cost of analog trunks, on both the ACD/PBX interface side and the outside telephone network side. One high-density 24-channel T1 phone set interface card often replaces three 8-channel analog cards in a digital ACD or PBX. New digital network technologies and increased competition among the long distance carriers are expected to result in even lower prices for digital network services.
Users of digital systems have heretofore employed analog bypass (denoted by dotted lines) by using a mixture of digital and analog trunks. All trunks are in service during normal operation, but only the analog trunks are operational during a power failure. Analog trunks must be terminated on analog-type telephones, often used solely for this back-up purpose. Thus in the prior art, providing back-up for digital telephone systems is expensive, and even then only limited back-up service is made available.